1. Field of the Invention
The present invention generally relates to a coaxial line shape resonator with a high dielectric constant element, and particularly to minimization of the resonator size by using a high dielectric constant element therein.
2. DESCRIPTION OF THE PRIOR ART
Hitherto, high dielectric constant elements suitable for use in resonance elements or high frequency filters have been made of high dielectric constant ceramic. FIG. 1 shows a prior coaxial line shape resonator, which comprises an inner conductor 1 of a tubular shape disposed in a central hole or aperture 5, a high dielectric constant ceramic body 3 formed around the inner conductor, and an outer conductor 2 formed on the outside faces of the high dielectric constant ceramic body 3 except for its top surface. The lower end of the inner conductor 1 is electrically connected to the bottom part of the outer conductor 2. The above-mentioned construction is contained in a metal case 4. In such a coaxial line shape resonator, the wavelength .lambda..sub.g of the electromagnetic wave in the resonator is given by the following equation: ##EQU1## wherein f is the frequency of the electromagnetic wave, L is the inductance per unit length of the coaxial line, C is the capacitance per unit length of the coaxial line, .epsilon. is the specific dielectric constant of the high dielectric constant ceramic body 2 and C.sub.0 is the capacitance per unit length of the coaxial line when there is no high dielectric constant body 2.
As shown by the above-mentioned equation, by inserting the high dielectric constant body 2 in the coaxial line shape resonator, the wavelength .lambda..sub.g in the coaxial line shape resonator can be shortened to 1/.sqroot..epsilon. times that for the coaxial line shape resonator without the high dielectric constant body 2. Therefore, the insertion of the high dielectric constant ceramic body in the cavity part of the resonator can effectively shorten its length. However, the high dielectric constant ceramic which is made by firing the raw ceramic material has great difficulty in working after the firing, and accordingly, adjustment of the resonance frequency by adjusting the size of the high dielectric constant ceramic body is very difficult. Also, the high dielectric constant ceramic body is expensive. Furthermore, the conventional high dielectric constant ceramic known in the prior art has a considerable temperature dependency in its .epsilon. value, therefore, the .epsilon. in reality is limited to a low value of about 40.